Method for the production of furfural from vegetable matter

ABSTRACT

Process and apparatus for producing furfural from plant materials. The hydrolysis of pentosanes contained in plant materials is effected in a first reactor (2) in the presence of a strong concentrated acid, at 20°-70° C., at atmospheric pressure, and dehydration of the pentoses into furfural is effected in a second reactor (5) by vapor action at atmospheric pressure and at a temperature lower or equal to 110° C., in a strong acid concentrated medium. Application: production of furfural.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel process for the preparation offurfural from vegetable matter and to the installation for carrying outthis process.

Furfural is a compound of the formula: ##STR1## which is of greatindustrial importance by reason of its numerous applications, notablyfor the manufacture of synthetic textile fibers, of plastics materials,of synthetic rubbers, etc . . . .

Furfural is obtained from vegetable matter containing pentosans, such asears of corn, oat, rice or cotton husks, by hydrolysis of the pentosansto obtain pentoses, which by dehydration, give furfural, in accordancewith the following reactions: ##STR2##

2. Description of the Prior Art

It has been proposed to manufacture furfural from vegetable mattercontaining pentosans by treatment of the latter, previously moistenedwith a non-volatile dilute acid, which plays the role of catalyst forthe degradation of the pentosans according to the reactions (1) and (2),by steam under relatively high pressure, of the order of 4.2 kg/cm².According to this technique, described in U.S. Pat. No. 1,735,084 ofSept. 7, 1922, treatment with steam under pressure effects at the sametime the degradation of the pentosans and the distillation of thefurfural resulting from this degradation. However, the yields offurfural obtained by this method are very low, so that the latter onlyhas slight economic interest for the production of furfural on anindustrial scale. This is why it has been sought to improve processesfor the production of furfural from vegetable matter containingpentosans, by subjecting the latter to steam treatment includingessentially two steps, as described in French Pat. No. 1,181,953 ofSept. 3, 1957. According to the process described in this French Patent,the raw material is subjected to a prior treatment with steam, at apressure close to atmospheric pressure, then to a second treatment stepby steam at a higher pressure, varying from 2.8 kg/cm² at the beginningof the operation to 7 kg/cm² at the end of the operation. Thenon-volatile acid used as catalyst may be sulfuric acid, phosphoric acidor an alkane-sulfonic acid and which can be added to the vegetablematter indifferently either in the second step of the treatment, or inthe prior treatment step. The steam utilized in super-heated to about270° C. at a pressure of 10.5 kg/cm² and the yield of furfural is 68% oftheory, which should correspond to about 16% by weight of furfural withrespect to the dry weight of the charge. The first step of the processmay proceed in a vessel of less robust construction than the digester,which must be adapted to withstand high temperatures and pressures, inwhich the second step of the process develops. This process however alsopresents numerous drawbacks, represented by:--the yields of furfuralthat it provides are still insufficient and--the application of the hightemperatures and steam pressures, which necessitate the use, in thecourse at least of the second step of the process, of special equipment,generally very expensive, adapted to withstand such temperatures andpressures, and have explosion risks.

There also exists a process called "Agrifurane Process" (cf. "TECHNIQUESDE L'INGENIEUR--Genie chimique", Vol 4, page J. 6020-1501) for theproduction of furfural by hydrolysis in an acid medium of vegetablematter rich in pentosans. This process effects the hydrolysis by theinjection of steam into steel reactors under a pressure of 10 bars. Thefurfuralized vapors which emerge from these reactors contain 5 to 6% byweight of furfural, so that it is necessary, to recover the technical90% furfural, not only to condense them but to subject them toazeotropic distillation, which is a relatively complicated and expensiveoperation. The yield of furfural obtained is of 10 to 13% with respectto the dry weight of the raw matter utilized. This process hence has thedrawback of applying high pressures, which necessitate the use ofreactors resistant to these pressures, and the yield of furfural whichit allows to be obtained is extremely low and can only be achieved atthe cost of treatments for the removal of the water which involvesrelatively expensive equipment and which are long and complicated.

It is consequently an object of the present invention to provide animproved process for the production of furfural, which responds betterto the necessities of practice than the processes proposed according tothe prior art, notably in that it is more economical than the processesof the prior art.

It is another object of the invention to provide a process for theproduction of furfural which does not have to resort to the applicationof high temperature and pressures.

It is a further object of the invention to provide a process for theproduction of furfural which permits the recovery and recycling of theacid used as catalyst.

It is yet another object of the invention to provide a process whichenables the yields of furfural production to be improved.

It is yet another object of the invention which provides a process whichcan be applied with relatively inexpensive equipment, and which does nothave to undergo stress imposed by high temperatures and pressures.

Other objects and advantages of the invention will appear from thedescription which follows.

GENERAL DESCRIPTION OF THE INVENTION

According to the present invention there is provided a process for theproduction of furfural from vegetable matter containing pentosans,characterised in that in the course of a first step, hydrolysis of thepentosans contained in the vegetable matter is carried out in thepresence of a concentrated strong acid, at atmospheric pressure, at atemperature of the order of 20° to 70° C., to obtain a solution ofpentoses which is dehydrated in the course of a second step, by theaction of steam at a pressure of 1 to 2 bars and at a temperature belowor equal to 110° C., in a concentrated acid medium, to give furfural.

According to an advantageous embodiment of the process according to thepresent invention, the two above steps are carried out in differentreactors.

In accordance with the invention, the furfural obtained is subjected toa suitable purification process, to obtain pure furfural.

According to a preferred embodiment of the process according to theinvention, the concentrated strong mineral acid in whose presence thehydrolysis of the vegetable matter is carried out is a volatile acid,preferably 5 to 6 N hydrochloric acid (azeotropic concentration at 20%by weight).

According to another preferred embodiment of the invention, thehydrolysis of the vegetable matter by a concentrated strong acid, iscarried out within the space of 1 to 2 hours.

When the duration of hydrolysis is limited to 1 to 2 hours, only thepentosans are degraded; if it is prolonged beyond these times, thecellulose of the vegetable matter is attacked by the concentrated strongacid, to give rise to sugars, and notably to glucose.

According to a particularly advantageous modality of the processaccording to the invention, the hydrolysis process is accelerated bysubjecting the reaction medium to stirring.

In accordance with the invention, this stirring is produced by recyclingthe acid solution of pentoses obtained in the course of the first step,into the reaction medium constituted by the vegetable matter and theconcentrated strong acid.

According to another advantageous embodiment of the process according tothe invention, the dehydration steam applied in the course of the secondstep of the process is at a temperature of about 100° to 110° C.

In accordance with the invention, the dehydration step is carried out bycounter-current circulation, in a reactor, of the pentose solution to bedehydrated and the dehydration steam.

According to a particularly advantageous feature of the invention, thepentose solution to be dehydrated and the dehydration steam areintroduced continuously in the dehydration step.

According to another particularly advantageous feature of the invention,the pentose solution to be dehydrated is admitted at the head of thereactor whence it flows by gravity, whereas the steam circulates incounter-current from the bottom of the reactor, thus permitting rapidcontinuous extraction, of the furfural formed and avoiding,consequently, any resinification reaction of the latter.

According to yet another advantageous feature of the invention, thepentose solution to be dehydrated is supplemented with a suitableantifoaming agent, which according to an advantageous modality of theinvention, may be a silicone based antifoaming agent.

According to another advantageous feature of the invention, at the endof the dehydration step, the concentrated acid is recovered by simpledecantation, to be recycled into the hydrolysis step.

According to an advantageous modality of the invention, the acidrecovered is subjected, prior to its recycling, to a distillationprocess to bring it back to its azeotropic composition of 20% by weight.

According to another advantageous feature of the invention, the residueresulting from the hydrolysis of the vegetable matter is separated bysimple heating, of the concentrated acid used for the hydrolysis, thatit contains, if the latter is a volatile acid, to be recovered and madereusable.

According to another aspect of the present invention there is providedan installation for the production of furfural by applying theabove-defined process, which installation is characterised in that itcomprises in combination:--at least one first reactor associated with anintake device for the vegetable matter and a concentrated strong acidstorage tank to which it is connected by a feed pipe;--at least onesecond reactor into which opens an inlet pipe for the pentose solutioncontaining the concentrated strong acid, coming from at least one firstreactor, and which comprises means for introducing steam at atemperature of the order of 100° to 110° C., a lower orifice for removalof the concentrated acid, which opens into the acid feed pipe of thefirst reactor and an upper orifice for the removal of the furfuralizedsteam to a condenser and a decanter whence the liquid furfural separatedis sent by suitable means to a storage tank.

According to an advantageous embodiment of the installation according tothe invention, at least one first reactor is associated with stirringmeans for the reaction mixture that it contains and which is essentiallyconstituted by vegetable matter and concentrated strong acid.

According to an advantageous feature of this embodiment, the above saidstirring means are constituted by recycling means for the acid solutionof pentoses, which recirculate the latter into at least one firstreactor.

The introduction of the vegetable matter into such a first reactor, aswell as removal of its residue, may be carried out continuously orsemi-continuously through lock orifices of known types.

According to another advantageous embodiment of the installationaccording to the invention, the one or more second reactor(s) is (orare) filled with a packing to improve the distribution and the contactof the reactants in the one or more said second reactor(s).

According to an advantageous feature of the invention, the one or moresecond reactor(s) is (or are) surrounded with heating means for said oneor more reactor(s) such as steam heating coils, for example, to limitthe condensation of the dehydration steam to the maximum.

According to an advantageous modality of the invention, the condensedwater recovered at the outlet of the condenser and of the decanter, issent to a boiler or the like whence it is recirculated, aftervaporization, in the steam inlet pipe into at least one second reactor.

According to the invention, the boiler or the like is preferably aboiler operating as a thermo-siphon.

According to the invention, the installation comprises in addition apurification installation for the furfural coming from the abovesaidstorage tank, in which the furfural is freed from the water that itcontains and which is connected, through a pipe to a storage tank foranhydrous furfural.

Also according to the invention, the dehydration acid removed from theone or more second reactor(s) is sent, prior to its recycling into atleast one first reactor, into a distillation column to extract therefromthe water (notably the water generated in the course of the reaction)and bring it back to its initial composition).

According to an advantageous feature of the invention, the installationcomprises, at the outlet of the at least one first reactor, acondenser-evaporator in which the sugars, and in particular the glucose,obtained by the acid attack of the cellulose of the vegetable matter,are separated from the acid removed from the at least one secondreactor, to be recovered at the outlet of said condenser-evaporator,prior to recirculating the acid into the at least one first reactor.

For the practising of the process of furfural production according tothe present invention, operation is preferably under the conditionsexplained below:

The raw material containing the pentosans utilized, is constituted byvarious vegetable scraps, such as corn cob, husks of oats, of rice, ofcotton or other residues of agricultural origin, or by vegetable matterwith a rigid structure, such as wood stumps, or again by sawmill scrapssuch as sawmill ends, shavings, sawdust and wooddust.

The concentrated strong acid applied in the process is, preferably, avolatile acid, such as 5-6 N hydrochloric acid.

The steam utilized in the second step of the process, namely thedehydration step of the pentoses obtained in the first step of theprocess, is steam at 100°-110° C., at a pressure of 1 to 2 bars, whichprocures a certain number of advantages, which will be explained below.

The process according to the present invention is applied according tothe flow sheet diagram shown in FIG. 1 of the accompanying drawings.

Into the first reactor are introduced the vegetable matter to be treatedas well as the concentrated strong acid, preferably volatile, at atemperature comprised between 20° and 70° C. and preferably comprisedbetween 30° and 60° C. The acid hydrolysis reaction of the pentosans ofthe vegetable matter, is effected in the space of 1 to 2 hours. If thecontact time between the acid and the vegetable matter is prolonged, thecellulose of the latter is then attacked by the acid, and is degraded togive sugars, and in particular glucose, which are recovered, byseparation of the acid in a suitable installation, prior to recyclingthe acid into the first reactor: it has been observed that a good yieldof glucose is obtained for an average dwell time between the acid andthe vegetable matter, of the order of 8 hours.

The hydrolysis of the pentosans gives rise to a pentose solution whichis sent into a second reactor, if necessary after passage in a heatexchanger, whilst the residue from the hydrolysis is recovered at theoutlet of the first reactor, to be made reusable, after having beentreated by heating to free it from the volatile acid which is separated,entraining with it the water possibly contained in the vegetable matter.This residue may be used as fuel, for example to produce the steamnecessary for the second step of the process, or as a source ofvegetable protein.

The second reactor is advantageously constituted by a column filled witha packing, for example of ceramic, which facilitates the contact and thedistribution of the reactants in said reactor.

In certain cases however, a progressive clogging of the packing of thissecond reactor in which the dehydration takes place occurs, by entrainedimpurities coming from the vegetable matter treated. In such a case, itis possible to filter the pentose solution by means of a filter mountedon the supply pipe of this second reactor. As a modification, it ispossible to replace the packing column by several, three for example,stirred second reactors mounted in series.

As the outlet of the first reactor, the pentose solution containing theconcentrated acid is sent to the upper part of the second reactor,whilst the steam at 100°-110° C. is introduced, at a pressure of 1 to 2bars, and preferably, at atmospheric pressure, at the base of saidsecond reactor. The steam containing furfural is disengaged from thesecond reactor to be led to a condenser connected to the upper part ofsaid second reactor. The furfuralized steam obtained at the outlet ofthe second reactor contains 30% of furfural, which constitutes aconsiderable advantage: in fact, due to the fact that thefurfuralization reaction is carried out at atmospheric pressure, thesteam which emerges from the furfuralization reactor is more chargedwith furfural than it could be in a furfuralization treatment which usesa pressure of the order of 10 bars, as is the case in the Agrifuraneprocess. This content of the order of 30% furfural in the steam, enablesthe furfural to be recovered by a simple condensation and decantationoperation and does not necessitate resorting to an azeotropicdistillation treatment, difficult to put into operation, with awkwardequipment.

After passage into the condenser and into a suitable decanter, thecondensed water is sent to a boiler, which operates preferably as athermo-siphon, in which it is vaporized, then recycled into the steaminlet pipe which feeds the second reactor with steam.

As its outlet from the decanter, the furfural is recovered in thestorage tank in the form of 90% furfural, after having been bubbled, asnecessary, in a neutralizer containing sodium carbonate and sodiumhydroxide to remove therefrom the traces of acid that it contains.

Although reference has been made in the foregoing, to a first reactor inwhich hydrolysis of the pentosans into pentoses takes place, and to asecond reactor in which the dehydration of the pentoses into furfuraltakes place, it is self-evident that the industrial installation maycomprise a plurality of first reactors mounted in series and/or inparallel and a battery of second reactors which are mounted in seriesand/or in parallel and in which the solution of pentoses and the steamcirculate successively in counter-current to improve the distributionand the steam/pentose solution contact and to complete the dehydrationreaction of the pentoses into furfural.

The 90% technical furfural may advantageously be subjected to anadditional treatment of purification to free it from the water that itstill contains at the end of the dehydration treatment, by distillationin a scrubbing column, such as a vacuum plate column, whence theanhydrous furfural is recovered to be sent into a storage tank, whilstthe water is recovered and freed, in a condenser and separator, from thetraces of furfural that it contains, which are sent to the distillationcolumn, whilst the water may be recovered by any suitable means to be ifnecessary recycled, after vaporization, into the steam inlet pipe in thesecond reactor.

The process according to the present invention has numerous advantageswith respect to the processes proposed in the prior art.

In fact, the realization of the process in two separate reactors, usingmoderate temperature conditions and substantially atmospheric pressure,minimizes the side-reactions which are produced in the processes of theprior art at the stage of the dehydration reaction, and notably theresinification of the furfural on its formation, the condensationreactions of the furfural, the oxidation of the furfural by the oxygencontained in the vegetable matter, the risk of these side reactionsbeing practically eliminated not only by the use of conditions oftemperature and of pressure according to the invention, but also by thefact that the dehydration of the pentoses takes place in a separatevessel from which the initially treated vegetable matter is absent andthat, in addition, the furfural can be separated from the liquid phase,in the form of furfuralized vapor, progressively with its formation,thus avoiding the interfering reactions resulting from prolonged contactbetween the furfural and the liquid phase.

The use, for the acid hydrolysis, of a concentrated strong acid, enablesa good hydrolysis yield to be obtained: it considerably reduces the timeof the reaction; moreover, in the dehydration step, the acid is notentrained by the steam and can thus be recycled to the hydrolysisreactor after possible separation of the sugars from the acid. Thisadditional advantage is due to the fact that the dehydration is carriedout at 100°-110° C., that is to say at a temperature below thevaporization temperature of the water-concentrated strong acid azeotrope(in the case where hydrochloric acid is used, the vaporizationtemperature of the water-HCl azeotrope is 110° C.), and that there is aconcentration of acid close to the azeotrope: 20% by weight of HCl. Tothis has been added, as stated above, the use of a volatile concentratedstrong acid, such as hydrochloric acid, which considerably facilitatesthe recovery and making reuse of the residues obtained after hydrolysissince this acid is very easily separated from the solid residue byheating, entraining the water possibly contained in the treatedvegetable matter.

The possibility of recycling the acid emerging from the one or moredehydration reactors, represents a very favorable economic factor in theapplication of the process according to the invention.

In addition, the almost total suppression of the interfering reactionsimproves considerably, consequently, the yield of furfural obtained.

On the other hand, the flexibility of the process according to theinvention, in which it is possible to select at will the time of theacid hydrolysis reaction, enables the composition of the hydrolysateobtained to be controlled and in particular to obtain either onlypentoses, or also other useful products such as sugars.

Another advantage of the process according to the invention resides inthe fact that the use of moderate temperatures and atmospheric pressureenables the development of the process according to the invention in arelatively inexpensive installation, whereas it has not to be subjectedto the stresses resulting from the use of high temperatures andpressures as is the case in the prior art. In addition, the energyinvestment is considerably reduced with respect to the processes of theprior art due to the fact that operation is at relatively lowtemperature and at atmospheric pressure, eliminating in addition therisks of heat losses due to the fact of the particularly favorableoperational conditions of the process.

In other respects, the safety of the installations is considerablyimproved with respect to the prior art and, in particular, the risk ofexplosion is eliminated due to the fact that operation is at atmosphericpressure.

On the other hand, the conditions of application of the processaccording to the invention enable continuous operation, at thedehydration stage, thus reducing the total duration of the furfuralproduction process with respect to the times necessary in the processesof the prior art.

Another important advantage of the process according to the invention isto remove the need for the azeotropic distillation necessary in theprocesses of the prior art, since the furfuralized steam which emergesfrom the furfuralization step contains about 30% of furfural.

In addition to the foregoing features, the invention comprises stillother features, which will emerge from the description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the additional descriptionwhich follows, with reference to the accompanying drawings in which:

FIG. 1 shows, as already mentioned, the flow diagram of the processaccording to the invention;

FIG. 2 shows, diagrammatically, one embodiment of a furfural productioninstallation, according to the invention, which refers, in addition, toan example of the practising of the process in an installation accordingto the invention;

FIG. 3 shows, diagrammatically, another embodiment of a furfuralproduction installation, according to the invention, and

FIG. 4 shows in the form of graphs the influence of temperature on thekinetics of the hydrolysis reaction, whilst

FIG. 5 shows the graphs of the molar yields of the furfuralisationreaction.

It must be well understood, however, that these drawings andcorresponding descriptive parts, in the same way as the example ofpractising the process, are given solely by way of illustration of theinvention of which they do not constitute a limitation in any way.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a flow diagram of the process according to the invention,in which the vegetable matter is introduced through a suitable device 1,into a first reactor 2 in which a concentrated strong acid is alsointroduced through a pipe 3. The concentrated strong acid, which isadvantageously 5.5 N hydrochloric acid, at 20% concentration, isintroduced into the reactor 2, at a temperature comprised between 20°and 70° C., and preferably, between 30° and 60° C. The hydrolysis takesplace in the reactor 2, at atmospheric pressure.

The pentose solution obtained by hydrolysis of the pentosans containedin the vegetable matter treated in the presence of the acid, is sentthrough the pipe 4 into a second reactor 5, in which it arrives,possibly after reheating in a heater 6. Although the pentose solutioncontaining the concentrated strong acid is introduced at the upper partof the reactor 5, the steam at 100°-110° C. is introduced at the base ofthe reactor 5, so that a counter-current circulation is establishedbetween the pentose solution to be dehydrated and the steam at 100°-110°C. The dehydration reaction of the pentoses takes place substantially atatmospheric pressure at the temperature of the steam, that is to say at100°-110° C.

The furfural is removed from the reactor 5, preferably at the upper partof the latter, in the form of furfuralised steam which is sent into acondensor, whilst the acid which constitutes the liquid phase, iswithdrawn at the base of reactor 5, to be recycled, if necessary aftercooling in the exchanger 6, into the hydrolysis reactor 2.

The vegetable residue contained at the end of the hydrolysis of thevegetable matter, is withdrawn from the reactor 2 to undergo heattreatment for removing the residual acid that it contains, with a viewto its recovery and reutilisation.

The furfural production installation shown by way of non-limitingexample in FIG. 2 is designed according to the flow diagram of operationshown in FIG. 1.

This installation comprises three units:

a unit in which the hydrolysis of the pentosans into pentoses iseffected;

a unit in which the pentoses are dehydrated to give furfural;

a scrubbing unit for the technical furfural obtained in the second step,to obtain anhydrous furfural.

The hydrolysis unit comprises two reactors 10 into each of which thevegetable matter to be treated is introduced by means of a suitabledevice, such as a belt conveyor 11 which cooperates with an elevator orthe like 12 which transfers the vegetable matter from the storagelocation 13 to the conveyor 11. The vegetable matter is advantageouslyintroduced by gravity into the reactors 10.

The concentrated strong acid coming from a storage tank 14 is introducedat a temperature of 20° to 70° C., and preferably at a temperature of30° to 60° C., into the reactors 10, through the pipe 15. The acidhydrolysis reaction of the pentosans contained in the vegetable mattertakes place in the reactors 10, at atmospheric pressure and at amoderate temperature advantageously comprised between 20° and 70° C.,and preferably comprised between 30° and 60° C.

Due to the fact of the use of a concentrated strong acid, the hydrolysisreaction is extremely effective and rapid; its duration is on theaverage from 1 to 2 hours. However, to the extent that it is desired toobtain not only furfural, but also sugars resulting from the attack onthe cellulose of the vegetable matter by the acid, it is possible toprolong the time of the reaction in the reactors 10. It appears that acontact time between the vegetable matter and the acid of the order of 8hours, enables a degradation of the cellulose to glucose to be produced,which is recovered as will be explained below. The duration of thehydrolysis reaction is also determined by the type of vegetable mattertreated: thus its duration will be longer in the case where vegetablematter with a rigid structure is processed, such as wood stumps, forexample.

At the outlet of the reactors 10, are collected, on the one handvegetable residue from the hydrolysis which is withdrawn through pipes16 and, on the other hand, the pentose solution containing theconcentrated strong acid. In the case where not only the hydrolysis ofthe pentosans is carried out, but also the degradation of the celluloseinto sugars, and notably into glucose, it is necessary to provide aseparating installation for the sugars from the attacking acid, whichinstallation may advantageously be constituted by anevaporator-condensor 32 mounted in by-pass on the pipe 42, at the outletof which the sugars, notably the glucose, are recovered (at 33), whilstthe acid is recirculated through the pipe 34, into the pipe 15 forsupplying the reactors 10 with acid.

The hydrolysis reaction is accelerated in the reactors 10 by subjectingthe reaction medium, constituted by the vegetable matter and the acid,to stirring. This stirring is advantageously produced by recirculationof the acid pentose solution obtained in the reaction medium: all orpart of said solution is taken up again by a pump 46 and recycled intoeach of the reactors 10 through a pipe 47.

The pentose solution containing the concentrated strong acid is sent, ifnecessary after bringing to suitable temperature in a heat exchanger 43,through a pipe 17 into a battery of columns 18, 19, 20 mounted inseries, containing a contact packing 21, of ceramic, for example.

Steam at 100°-110° C. is also sent through a pipe 22 into the battery ofcolumns 18, 19, 20. Whereas the pentose solution is introducedsuccessively at the upper part of the columns 18, 19, 20 through pipes17, 23 and 24, the steam at 100°-110° C. is introduced at atmosphericpressure at the base of the column 20, in which it circulates incounter-current to the pentose solution; it thus converts the pentosesby dehydration into furfural which it entrains on its passagesuccessively in the columns, namely 19 through the pipe 25, and 18through the pipe 26: the furfuralised steam is removed at the top of thecolumn 18 through a pipe 27; these furfuralised vapors contain 30% offurfural due to the fact that the treatment is carried out atatmospheric pressure, thus enabling the vapors to receive a greatercharge of furfural than is the case in processing installation of theprior art, in which the pressure applied, which is generally of theorder of 10 bars, prevents the vapors from being charged with furfuralto a proportion higher than 5 to 6%. Due to the fact of their highcontent of furfural, the furfuralised vapors do not have to undergo, toenable the recovery of the furfural, an azeotropic distillationtreatment as is the case in the prior art; a simple treatment by passagein a condensor 28, and then a decantor 29, suffices to recover theliquid furfural; the liquid furfural, decanted in the decanter 29, issent to a storage tank 30 if necessary after neutralisation, thecondensed water being sent from the decanter 29 into a boilor 31 whereit is vaporised to be recycled in the pipe 22 for supplying the column20 with steam at 100°-110° C. The hydrochloric acid is withdrawn at thebase of the column 20 whence it is recycled through the pipe 42 into thepipe 15 for supplying the reactors 10 with acid, after having beencooled to a temperature of 20°-70° C. and preferably to 30°-60° C. in aheat exchanger 43 and after being freed from the sugars that itcontains, in the evaporator-condenser 32, as indicated above.

The furfural collected in the tank 30 is a 90% technical furfural fromwhich it is necessary to eliminate the water present as impurity, to theextent that it is desired to obtain anhydrous furfural. Thispurification step is carried out by circulation of the 90% technicalfurfural solution into a purification installation known in itself, suchas a plate distillation column 37. The furfural is introduced throughthe pipe 35 into said column at about 50° C., under a vacuum of 0.1 bar.The anhydrous furfural obtained is withdrawn through a pipe 37 to besent to storage tanks 44, whilst the water separated from the furfuraland containing a small amount of dissolved furfural is sent into acondenser 38, then a separator 39 whence the decanted furfural is sentback into the column 36 through a pipe 40, whilst the water is removed,through a pipe 45, to an ejector 41 which sends it, if necessary, by anysuitable means to the boiler 31 where it is vaporised, and then recycledinto the pipe 22.

EXAMPLE OF THE PRACTISING OF THE FURFURAL PRODUCTION PROCESS ACCORDINGTO THE INVENTION

Corn cobs with 12% of moisture are introduced at the rate of 1.95ton/hour into each of the reactors 10, by means of the belt conveyer 11.The 5.5 N hydrochloric acid at 20% azeotropic concentration at atemperature of 40° C., coming from the tank 14, is introduced throughthe pipe 15 into each of the reactors 10.

The duration of the acid hydrolysis, which is carried out in thereactors 10 at a pressure of 1 bar and at a temperature of 40° C., isabout 8 hours. The duration of hydrolysis of the pentosans contained inthe vegetable matter into pentoses is only 1 hour, under the conditionsof the reaction. The prolongation of the contact time of the vegetablematter with the hydrochloric acid used as hydrolysis catalyst, up to 8hours, causes the attack of the cellulose of the vegetable matter andits degredation to the stage of glucose. The pentose solution obtainedwhich contains hydrochloric acid and glucose is recycled at least inpart by means of the pump 46 and the pipe 47 to each of the reactors 10,to cause stirring in the reactor and accelerate on the one hand thehydrolysis and on the other hand the degradation of the cellulose. Thesolution of pentoses which contains the hydrochloric acid, leaves thereactors 10 through the pipe 17 to the furfuralisation step. Thevegetable residue from the hydrolysis is withdrawn from the reactors 10through the pipe 16, at the rate of 1.45 ton/hour, to suitableprocessing units.

The acid solution of pentoses is sent, through the pipe 17, into abattery of packed columns 18, 19, 20 at the same time as the steam at105° C. and at a pressure of 1 to 1.3 bars, is introduced incounter-current in the battery of columns 20, 19, 18.

At their outlet from the battery of columns 18, 19, 20, the furfuralizedvapors at 100° C., containing 30% of furfural, are treated in acondenser, then in a decanter to recover the 90% technical furfural andthe water which is vaporized and recycled in the form of steam into thebattery of columns 20, 19, 18.

The hydrochloric acid is withdrawn, at the rate of 13 m³ /hour, at thebase of the column 20 to be recycled into the circuit 15 supplying acidto the reactors 10, if necessary after restoring to the temperature of40° C. in the exchanger 43. Before being recycled into the acid supplycircuit 15 of the reactors 10, the hydrochloric acid from the pipe 42 issent, through a branch line, into an evaporator-condenser 32 in which itis freed from the glucose that it contains, which is recovered at 33,for its use and/or possible rendering of commercial value.

Anhydrous furfural at the rate of 250 kg/hour is obtained by treatmentof a 90% technical furfural obtained at the outlet from the dehydrationstage, in a vacuum purification installation (0.1 bar) at 50° C.comprising essentially a plate distillation column or the like asdescribed above with respect to FIG. 2.

The furfural production installation shown by way of non-limitingexample in FIG. 3 is designed, in the same way as the installation inFIG. 2, according to the operating flow diagram shown in FIG. 1.

In the installation of FIG. 3, the unit in which the hydrolysis of thepentosans into pentoses is carried out comprises a single reactor 48, inwhich the vegetable matter to be treated is introduced continuously bymeans of a suitable device such as that described with reference to FIG.2.

The concentrated strong acid coming from a storage tank 49 is introducedcontinuously at a temperature of 60° C., into the reactor 48 through thepipe 50, in counter-current to the vegetable matter. The acid hydrolysisreaction of the pentosans contained in the vegetable matter takes placein the reactor 48, at atmospheric pressure and at a moderatetemperature, of the order of 60° C., in the space of 1 to 20 hours. Thevegetable residue from the hydrolysis, is collected, at the base of thereactor 48, through a reaction lock chamber 87 for example, on a bandfilter 51, where it undergoes draining, and whence it is sent into awater washing column 52, to extract the acid therefrom; then it istransferred into a screw press 53 in which it is dehydrated before beingburnt and gasified in a burner kiln 54 to provide steam, stored in thecontainer 55, designed to be used for the furfuralization.

The pentose solution containing the concentrated strong acid iscollected at the head of the reactor 48 and is sent through the pipe 56into a furfuralization reactor 57, after having been brought to suitabletemperature in a heat exchanger 58, and if necessary supplemented withan antifoaming agent. The furfuralization column 57 may if necessarycontain a contact packing of suitable material. Steam at 110° C. isintroduced into the base of the furfuralization column 57, incounter-current with the pentose solution to be converted into furfural,under a pressure of 1.3 bars. Rather than introduce the steam directlyinto the column 57, as in the embodiment shown in FIG. 2, in theinstallation shown diagrammatically in FIG. 3, the solution, run bygravity to the base of the column 57, is recirculated, by means of apump 59 into a thermo-siphon boiler 60 in which a part of the liquid isvaporized, the steam formed being injected through the pipe 61 to thebase of the column 57.

The furfuralized steam emerges at the head of the column with 30% byweight of furfural. After condensation and cooling in the condenser 62,the water-furfural mixture is separated into two phases in the separator65:

a phase with 95% by weight of furfural which is sent, through the pipe63, into the dehydration column 64;

a phase with 8% by weight of furfural which is reinjected by means ofthe pump 66 into the furfuralization column 57.

Preferably, the phase with 95% by weight of furfural is subjected,before being introduced into the dehydration column 64, to aneutralization process, preferably by means of Na₂ CO₃, in a reactor 67whence it is withdrawn into the vat 68.

The concentrated strong acid is withdrawn at the base of thefurfuralization column 57 for recycling into the hydrolysis reactor 48.However, taking into account the residual moisture of the vegetablematter treated by the acid in the reactor 48 and the water generated inthe course of the furfuralization reaction, the titer of acid has atendency to be below its initial composition, at the outlet of thecolumn 57. It is hence opportune to bring back the acid to its initialtiter before recycling it into the reactor 48. Such rectification iscarried out in a distillation column 69 into which the dilute acidwithdrawn from the column 57 is led through the pipe 70. There iscollected:

at the head of the column 69 the residual water and the possiblevolatile substances entrained by the acid at the outlet of the column 57(such as methanol)

at the bottom of the column 69, the rectified acid ready to be recycledinto the reactor 48 through the pipe 71 (by passing through the storagetank 49 and the pipe 50).

The distillation column 69 processes not only the acid withdrawn at itsoutlet from the furfuralization column 64, but advantageously also theacid extracted from the vegetable residue at its outlet from the reactor48, which is brought to it through the pipe 73 coming from the tank 72.

The phase with 95% by weight of furfural is introduced, through the pipe74, into the dehydration column 64 which operates under a vacuum of 100mm Hg, at 100° C. The column 64 is a plate distillation column in whichthe furfural coming from the furfuralization column, which is a 95%technical furfural, is dehydrated to remove therefrom the water presentas impurity. The 99% furfural obtained at the outlet of the distillationcolumn 64 is removed from the latter by means of the pump 75, to be sentinto the receiving vat 76 then, by means of the pump 77 into theanhydrous furfural storage vat 78.

The water separated from the furfural and containing a small amount ofdissolved furfural, is removed at the head of the distillation column 64to be sent into a condenser 79, then a separator 80 whence the decantedfurfural is sent back into the column 64 through the pipe 81, whilst thewater is removed to an ejector 82 from which it can be sent into thethermo-siphon boiler 60 where it is vaporized, then recycled into thefurfuralization column 57.

Just as the vegetable residue from the hydrolysis is incinerated toprovide steam for the furfuralization step, thus rendering the processautonomous in energy, the impurities withdrawn at the foot (83) of thefurfuralization column 57 are also sent to the incinerator 54 to beburnt and provide the steam useful in the process, and the impuritiescoming from the hydrolysis of the vegetable matter recovered at the footof the column 69 for regenerating the concentrated strong acid ofhydrolysis are sent, after neutralization in the reactor 85 andseparation in the decanter 86, to the incinerator 54 where they are alsoburnt to provide steam to the process.

There will be described below, by way of non-limiting example theapplication of the installation shown diagrammatically in FIG. 3, to theproduction of 5000 t/year of furfural, from 35,000 t/year of corn cobswith 30% moisture (it being however understood that the processaccording to the invention applies with the same advantages to theproduction of furfural from vegetable matter rich in pentosans, with alesser moisture content), the weight of dry matter being hence of theorder of 25,000 t/year and its composition being:

32% of pentosans

50% of cellulose

18% of lignin.

The hydrolyser 48 whose volume is 40 m³ is supplied with corn cobs atthe rate of about 3.12 t/hour of dry matter and with 20% hydrochloricacid at 60° C., at the rate of 9.36 m³ /hour. The flow rate of thehydrolysate solution at the outlet 56 of the hydrolyser 48 is 6.55 m³/hour and the concentration of pentoses of the hydrolysate is 150g/liter.

The hydrolysate solution containing 150 g/liter of pentoses enters thefurfuralization column 57 whose useful volume is 14 m³ at a flow rate of6.55 m³ /hour. The steam is injected at 110° C., at a pressure of 1.3bars, at a flow rate of 1.5 t/hour. The furfuralized steam which emergesfrom the column 57 is condensed in the condenser 62 to give notably awater-furfural mixture with 95% by weight of furfural (and aconcentration of pentoses of 8 g/liter). Under these conditions, theflow rate of the recycling pump 59 into the boiler 60 is 27 m³ /hour.

The regeneration of the hydrolysis acid, such as hydrochloric acid, atthe outlet of the furfuralization column 57 and prior to its recyclinginto the hydrolyser 48, is carried out in the distillation column 69,which is preferably a plate column. The column 69 is supplied by ahydrochloric acid solution coming:

from the extraction of the vegetable residue at its outlet from thehydrolyser 48, collected in the tank 72: 3.46 T/hour with 13% HCl

from the furfuralization: 8.56 T/hour with 17% HCl namely, in total,11.7 T/hour with 15.8% HCl by weight, with a supply flow rate of 12T/hour at 14.8% of HCl by weight. The withdrawal flow rate of the HClregenerated at the bottom of the column is 9.3 T/hour of HCl at 20%azeotropic concentration.

The outlet flow rate of the residual water at the head of the column is2.42 T/hour and the impurities flow rate at the foot of the column is0.3 T/hour.

The HCl loss is 1% with respect to the supply HCl, namely, on a 5,000T/year furfural unit,

at the furfuralization level: 1.4×0.01=0.014 T/hour of HCl

at the level of withdrawal of the impurities: 0.3 T/hour×0.2=0.06 T/hourof HCL, namely 0.4 T/T of furfural.

The mixture with 95% by weight of furfural is sent after decantationinto the decanter 65 and neutralization into the reactor 67, at a supplyflow rate of 0.67 T/hour, into a dehydration column 64 which operatesunder a vacuum of 100 mm Hg, to obtain at the outlet of the column 64, a99% by weight furfural, with an outlet flow rate of 0.65 T/hour, afterhaving extracted about 0.04 T/hour of water. The 99% furfural isobtained at the rate of 625 kg/hour.

The total consumption of the installation in steam is 7 T/hour and thetotal consumption of water 185 m³ /hour.

The combustion of the 2.12 T/hour (in dry matter) of vegetable residuecoming from the hydrolyser 48--with a PCI of 3,400 kcal/kg--produces7,200 Th/h, namely 11 T/hour of steam, which is hence supplied in largeexcess with respect to the requirements of the installation (about 7T/hour).

The influence of temperature on the kinetics of the hydrolysis reactionof the pentosans into pentoses has been determined by studying thedevelopment of pentose in the closed hydrolysis reactor, as a functionof the times for several temperatures.

The conditions of these tests are assembled in Table I below:

                                      TABLE I                                     __________________________________________________________________________    TESTS OF THE HYDROLYSIS OF VEGETABLE MATTER RICH IN PENTOSANS                 INTO PENTOSES                                                                           Cobs used: corn cobs dried in Crib - composition:                                                    11% moisture by weight                                                        38% of pentosans by weight                             density:               160 kg/m.sup.3                                                                     Maximum con-                                      Weight of         Volume of centration                                        crude cobs                                                                          Weight of   20% acid                                                                            Total                                                                             of pentoses                                       in the                                                                              dry   Weight of                                                                           in the                                                                              liquid                                                                            in the hydro-                             no.Test                                                                          (C.)Temperature                                                                     (kg)hydrolyser                                                                      (kg)matter                                                                          (kg)pentosans                                                                       (l)hydrolyser                                                                       (l)volume                                                                         (g/l)lyser                                                                            ##STR3##                       __________________________________________________________________________     1  23    45.6  40.5  15.4  173   178  99     0.26                             2  47    40.4  36    13.6  150   154.4                                                                             100     0.27                             3  60    40    35.6  13.5  175   179.4                                                                              85     0.22                            __________________________________________________________________________

The change in concentration of pentose in the hydrolyser as a functionof temperature, emerges from the graphs of FIG. 4 in which:

graph (1) shows the development at 23° C.

graph (2) shows the development at 47° C.

graph (3) shows the development at 60° C.

These tests permit two phenomena to be demonstrated:

the appearance of a pentosan-pentose equilibrium which slows down thehydrolysis kinetics and which prevents the maximum concentration ofpentoses to be reached in the closed reactor,

the appearance of a degradation reaction for the temperature of 60° C.and a duration of hydrolysis of about 5 hours and enable the enthalpy ofthe hydrolysis reaction to be calculated:

    ΔH=15.5k cal/mol

Study of the influence of the dwell time of the hydrolysate in thefurfuralization column on the yield of furfural has given the followingresults, which have been translated into graphs in the accompanying FIG.5:

the yield of furfural increases as a function of the dwell time in thefurfuralization column. The optimum is obtained when the dwell time isadapted to the kinetics for the production of the furfural: cf. graph(1) of FIG. 5 where dt=2 hours;

the yield of furfural decreases for high dwell times (dt>2 hours); theinterfering reactions are no longer negligible.

The graph 2 gives the molar yield furfural+pentose obtained per mole ofpentose injected and the graph 3 gives the level of pentoses unconvertedinto furfural. These graphs enable the excellent yield of furfural whichcan be reached by the process according to the present invention, to beverified.

It results from the foregoing description that, whatever the modes ofpractising, the embodiments and applications adopted, processes andinstallations for the production of furfural from vegetable matter areprovided, having with respect to earlier processes and installations forthe same purpose, important advantages of which certain have beenmentioned in the foregoing and which others will emerge from theutilization of said processes and installations.

Thus as emerges from the foregoing, the invention is in no way limitedto those of its methods of practice, embodiments and applications whichhave just been described more explicitly; it encompasses on the contraryall modifications which may come to the spirit of the technician in theart, without departing from the scope, nor the spirit, of the presentinvention.

I claim:
 1. A process for the production of furfural from vegetablematerial containing pentosans, comprising:(a) in a first reactor,hydrolyzing pentosans present in said vegetable matter in the presenceof a concentrated strong acid at or near atmospheric pressure at atemperature of about 20° to 70° C., thereby obtaining a solution ofpentosis; and (b) in a second reactor, dehydrating said solution ofpentoses by the action of steam applied at a pressure of about 1 to 2bars absolute and at a temperature up to 110° C. in a concentrated acidmedium, thereby yielding furfural.
 2. The process of claim 1, whereinsaid concentrated strong acid in said first reactor is a volatile acid.3. The process of claim 1, wherein said volatile concentrated strongacid is 5 to 6 N hydrochloric acid at 20% by weight azeotropicconcentration.
 4. The process of claim 1, wherein the hydrolysis of thevegetable matter with the concentrated strong acid is conducted withinthe period of 1 to 2 hours.
 5. The process of claim 1, wherein thecontact time between the vegetable matter and the concentrated strongacid is prolonged up to 4 to 12 hours, thereby causing degradation ofthe cellulose of the vegetable matter to sugars, notably glucose, whichare recovered.
 6. The process of claim 1, wherein the hydrolysis step isaccelerated by subjecting the reaction medium to stirring.
 7. Theprocess of claim 6, wherein the reaction medium containing saidvegetable matter and the concentrated strong acid is stirred byrecycling the acid pentose solution obtained from said first step. 8.The process of claim 1, wherein said steam is injected into said secondreactor during the dehydration reaction at a temperature of about 100°to 110° C.
 9. The process of claim 1, wherein the dehydration step isconducted by counter-current circulation, in said second reactor, of thepentose solution to be dehydrated and said steam.
 10. The process ofclaim 9, wherein the pentose solution to be dehydrated and said steamare introduced continuously into said second reactor.
 11. The process ofclaim 10, wherein the pentose solution to be dehydrated is admitted atthe head of said second reactor from whence it flows downwardly bygravity, while the steam circulates counter-currently from the bottom ofthe reactor, thus enabling rapid continuous extraction of the furfuralformed, and avoiding consequently, any resinification reaction of saidfurfural.
 12. The process of claim 9, wherein said steam introduced intothe dehydration step is obtained by the combustion and gasification ofvegetable residue obtained from the hydrolysis step.
 13. The processaccording to claim 9, wherein said steam introduced into the dehydrationstep is obtained by vaporization of a portion of the pentose solutionintroduced into the dehydration step.
 14. The process of claim 1,wherein the pentose solution to be dehydrated is supplemented with anantifoaming agent.
 15. The process of claim 1, wherein, at the end ofthe dehydration step, the concentrated acid is recovered by simpledecantation and recycled to the hydrolysis step.
 16. The process ofclaim 1, wherein the acid recovered at the end of the dehydration stepis subjected, prior to its recycling, to distillation in order to adjustits concentration to a 20% by weight azeotropic composition.
 17. Theprocess of claim 1, wherein the residue resulting from the hydrolysis ofthe vegetable matter is separated by heating said residue to recoversaid acid and is then reused.
 18. The process of claim 1, wherein thefurfural product obtained upon dehydration is subjected to apurification process to obtain pure furfural.
 19. The process of claim18, wherein the furfural product obtained from said dehydration step ispurified by distillation by the application of a vacuum on the order of0.1 to 0.3 bars absolute at a temperature up to about 100°-110° C.